The crystal of magnesium hydroxide belongs to hexagonal system and is generally platy in appearance with its thickness as a c-axis direction and its width as an a-axis direction because crystal growth in the a-axis direction differs from crystal growth in the c-axis direction.
Conventional magnesium hydroxide has a thickness of about 0.01 to 1.0 μm, a width of 0.01 to 1 μm and an aspect ratio of about 2 to 6.
Therefore, the conventional magnesium hydroxide is used as an antiacid (stomach antacid), a stabilizer for vinyl chloride, a laxative agent, a flu-gas desulfurizing agent, a magnesia fertilizer or a food additive (magnesium reinforcement), making use of its chemical properties and as a flame retardant for resins (use of heat absorption properties at the time of thermal decomposition), making use of its physical properties.
Magnesium hydroxide is a rare substance having the highest nontoxic level and inexpensive as its raw material is seawater rich in resources or underground water. However, the number of its uses is small. Therefore, the development of its new use by providing a new function is worthwhile environmentally and economically.
The inventors of the present invention already invented magnesium hydroxide whose crystal grows well and which is almost monodisperse (almost free from secondary agglomeration) and proposed its new use as a flame retardant for resins (JP-A 52-115799), and the magnesium hydroxide is now widely used. This has no safety problem at all as compared with other flame retardants such as organic halides and phosphoric acid esters all of which have problems such as toxicity.
However, magnesium hydroxide must be used in an amount of about 170 parts or more by weight based on 100 parts by weight of a resin, which degrades the physical properties such as mechanical strength of the resin. Therefore, novel magnesium hydroxide which does not degrade flame retardancy even with a much smaller amount thereof has been desired for a long time.